1. Field of the Invention
Essentially all of the oil from the more than 30 million tons of soybeans as well as millions of tons of other oilseeds and grain commodities crushed annually for use in the United States is extracted with hexane. Problems associated with the use of this solvent are numerous. It is highly flammable and explosive when mixed with air. It is also not selective for triglycerides resulting in the concurrent extraction of undesirable impurities including free fatty acids, phospholipids, pigments, and unsaponifiables. Significant refining losses and attendant energy requirements to remove such impurities contribute to production costs. Moreover, with the rapidly escalating costs, uncertain availability, and potential health and safety-related problems of hydrocarbon solvents in general, the social and economic environment has stirred interest in alternatives.
Supercritical fluids technology may be a viable alternative to current extraction methods. Supercritical fluids (SCF's) are often referred to as dense gases. Technically, an SCF is a gas existing above its critical temperature and critical pressure, as defined in the phase diagram of the pure substance. When a gas is compressed above its critical temperature, densities increase dramatically. Therefore, under a given set of conditions, an SCF may possess the density of a liquid while maintaining the diffusivity of a gas. The solvent properties of SCF's have been recognized for over 100 years but commercial applications have been slow in developing. As far as is known, decaffeination of coffee with supercritical carbon dioxide (SC-CO.sub.2) is the only current large-scale commercial SCF process.
SC-CO.sub.2 is an ideal solvent because it is nontoxic, nonexplosive, cheap, readily available, and easily removed from the extracted products. Moreover, preliminary unpublished results of Friedrich et al. indicate that the levels of pigments, iron, and phosphorus extracted from soybeans by SC-CO.sub.2 are significantly less than those extracted by hexane.
2. Description of the Prior Art
SC-CO.sub.2 has been investigated as an extracting solvent in the processing of a vast array of oleaginous products. One of the first teachings of this sort is found in British Pat. No. 1,356,749 wherein a variety of vegetable oils were separated with SC-CO.sub.2 from their respective seeds. Similar applications include the extraction of fats and oils from cocoa solids (British Pat. No. 1,356,750), starch-containing vegetable materials (Schwengers, U.S. Pat. No. 3,939,281), chamomile (Stahl et al., German Pat. No. 2,709,033), animal products (U.S. Pat. No. 4,280,961), and lupine seeds [Stahl et al., Fette, Seifen, Anstrichmittel 83(12): 472-474 (1981)]. Additionally, Caragay et al. [presentation at the 72nd Annual Meeting of American Oil Chemists Society, New Orleans, LA (May 18, 1981)] urges that SC-CO.sub.2 can be used to effectively fractionate and purify heat-abused oxidized cooking oils.
In a review paper by Brunner et al. entitled "The State of the Art of Extraction with Compressed Gases" (lecture presented at Annual Meeting of Process Engineers, Strassburg, Oct. 1-3, 1980), typical temperature and pressure parameters for extraction of seeds oils with supercritical CO.sub.2 were 130.degree. C. at 150 bar, 40.degree. C. at 220 bar, and 50.degree. C. at 350 bar. The paper also states that the solubility of a substance in a supercritical gas depends primarily on the density of the gas. Likewise, Johnston et al. [AIChE Journal 27(5): 773-779 (September 1981)] concludes that solubility correlates best with density as an independent variable and is in fact linear on a plot of log solubility vs. solvent density. According to Hubert et al. ("Extraction with Supercritical Gases," G. M. Schneider, E. Stahl, & G. Wilke, eds., Verlag Chemie, Deerfield Beach, FL, 1980, pp. 27-28) extraction with supercritical CO.sub.2 offers the advantages of being able to vary the density and dielectric constants by varying the pressure. This reference shows graphically that the greatest change in density and dielectric constant at 50.degree. C. take place from 70 to 150 bar and that both parameters increase only slightly above about 300 bar. It is clear therefrom that the increased solubility to be expected by employing CO.sub.2 at pressures much in excess of 300 bar would not justify the additional energy required. Furthermore, Stahl et al. (Fette . . . , supra), teaches in Table I that increasing temperature at constant pressure reduces the solubility of triglyceride in CO.sub.2. In accord with that observation, the preponderance of the art concurs that for oils and other low volatility substances the solubility in SC-CO.sub.2 is maximized at temperatures slightly above the critical point. The 3% by weight solubility of sunflower seed oil reported by Stahl et al. [J. Agr. Food Chem. 28(6): 1153-1157 (1980)] at 40.degree. C. and 700 bar is typical of the results obtained by prior art processes. In light of these observations, prior disclosures of extreme conditions (up to 2000 excess atmospheres and 170.degree. C., U.S. Pat. No. 3,843,824) have been no more than academic curiosities. With a plurality of polar compounds, Stahl et al. [Agnew. Chem. Int. Ed. Engl. 17: 731-738 (1978)] shows in FIG. 11 a decelerating rate of solubility increase as a function of pressure up to about 2500 bar at 40.degree. C.